Publication 82

Magnetotelluric response and geoelectric structure of the Great Slave Lake shear zone

Wu, X, I.J. Ferguson and A.G. Jones

Abstract

The Great Slave Lake shear zone (GSLsz) is a northeast-trending 25-km-wide dextral continental transform fault from the foothills of the Rocky Mountains in northeast British Columbia to the southeast side of Great Slave Lake. Based on its magnetic expression the GSLsz can be correlated for at least 1300 km, mostly in the subsurface. Magnetotelluric (MT) soundings were made at 60 sites in the southwest part of the Northwest Territories, Canada, along the LITHOPROBE Slave - Northern Cordillera Crustal Evolution (SNORCLE) Transect Corridors 1 and 1A, in the Summer and Autumn of 1996. Of these, 15 sites were along Corridor 1A which crossed the GSLsz, the Great Bear Magmatic Arc and Hay River terrane to the northwest of the shear zone, and the Buffalo Head terrane to the southeast. The primary objective of the Corridor 1A deployments was to image the structure of the GSLsz.

Analysis of MT data indicates that along the Corridor 1A the resistivity structure is approximately one-dimensional at shallow depth (<1 km) corresponding to Phanerozoic sedimentary rocks, two-dimensional in the upper to mid crust with a strike ~N30oE, and approximately two-dimensional in the lower crust to lithospheric mantle with a strike of ~N60oE. The direction in the upper crust is interpreted to represent the local-scale (<50 km) horizontal strike of the GSLsz whereas the direction in the mantle is parallel to the larger-scale strike of the GSLsz. Two-dimensional inversions of the MT data reveal that the GSLsz forms a crustal-scale resistive zone (>5000 Wm) that is spatially correlated with a magnetic low. The GSLsz comprises greenschist to granulite facies mylonites. Its high resistivity is interpreted to be due to the resistive nature of the granitic protolith of the mylonites and that mylonites are dominated by rocks deformed in the ductile regime.

To the northwest of GSLsz the MT profile reveals crustal conductors beneath the Great Bear Magmatic Arc and Hay River terrane. The enhanced conductivity occurring beneath the Great Bear Magmatic Arc is interpreted to be caused by electronic conduction within deformed and metamorphosed rocks of the Hottah terrane or the Coronation Supergroup. The MT results also reveal a mantle conductor beneath the margin of the Hottah terrane and Great Bear Magmatic arc that is interpreted to be associated with the subduction of oceanic lithosphere. A second mantle conductor to the southeast is truncated at the GSLsz suggesting an older source for the enhanced conductivity and that the GSLsz included significant strike-slip motion of sub-crustal lithosphere.

Source

Earth, Planet. Sci. Lett., 196, 35-50, 2002.

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Alan G Jones / 10 June 2004 / alan-at-cp.dias.ie